Psst - OpenVZ is awesome! I love the flexibility of it vs Xen and VMWare - being able to dynamically allocate all resources such as RAM comes in handy. You can even do a "zero downtime" migration of a container from one node to another. Check it out!

Uhm, Xen and VMWare (as well as KVM and probably Hyper-V) can do "Live Migration" as well. KVM can even do liver migration from an Intel-based server to an AMD-based server (and vice versa).

Xen and VMWare also support memory ballooning (live re-allocation of memory between guests).

The big difference between OpenVZ (and FreeBSD Jails, and Solaris Zones) and Xen/KVM/VMWare is that all your containers run using the kernel of the host OS whereas Xen/KVM/VMWare have separate kernels running in each VM. There are pros and cons to each method, depending on what you need to do.